Forager Success Increases with Experience in Polybia Occidentalis (Hymenoptera: Vespidae)
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Ins. Soc. 39:451-454 (1992) 1015-1621/92/040451-04 $ 1.50 + 0.20/0 C(d 1992 Birkhiiuser Verlag, Basel Short communication Forager success increases with experience in Polybia occidentalis (Hymenoptera: Vespidae) S. O'Donnell and R. L. Jeanne Department of Entomology, University of Wisconsin, Madison, WI 53706 U.S.A. Key words: Foraging, reliability, task performance, wasps. Summary Foragers of the neotropical swarm-founding wasp Polybia occidentalis showed improved task performance, as indicated by foraging success rate, with foraging age. Foragers also spent significantly more time in the field on foraging trips as they aged, while foraging rate did not change with age. These patterns were not explained by directional changes in resource availability or colony need over time. We compare these results to earlier findings on changes in task performance with experience in social insect foragers, and suggest that increases in forager persistence in the field explain improved foraging success with experience. Introduction Workers of many social insect species exhibit long-term behavioral specialization involving bias towards or exclusive performance of certain tasks. The assumption that task performance improves with experience is a key component ofevolutionary models of division of labor which posit an adaptive value for worker specialization (Oster & Wilson 1978). This assumption has rarely been tested empirically; two studies to date have demonstrated an improvement in task performance with experience in social insects. Heinrich (1976) showed that bumble bee foragers' abilities to handle particular types of flowers improved with practice, so that the handling time needed to extract nectar decreased as workers repeated visits to a particular flower species. Schmid-Hempel and Schmid-Hempel (1984) found that the success rate of prey capture by desert ant foragers increased with foraging age, and that experienced foragers spent more time away from the nest on forays. We tested the hypothesis that task performance by foragers of the tropical social wasp Polybia occidentalis improves with experience. P. occidentalis workers exhibit strong age polyethism and task specialization: foraging appears late in the age sequence of tasks, and foragers rarely switch to other tasks (Jeanne et aL 1988). We predicted that a measure of performance, the proportion of trips on which foragers successfully returned with materials, would increase with foraging age if task performance improved with experience. 452 O'Donnell and Jeanne Materials and methods Field work was conducted during the rainy seasons of 1989 and 1991 at Hacienda la Pacifica near the town of Canas in northwestern Costa Rica. Data were collected on individually marked, known-age workers introduced into observation colonies in the field (see O'Donnell & Jeanne (1992) for marking and introduction techniques). For aging age was defined as the number of days that had elapsed since a worker was first observed foraging. In 1991 we recorded the occurrence of successful and unsuccessful foraging trips from five colonies. We recorded a trip as unsuccessful when a forager landed, remained visible for more than 5 seconds before entering or leaving the nest, and was contacted by one or more nestmates, but did not transfer materials (P. occidentalis foragers usually transfer the loads they collect to nestmates: O'Donnell & Jeanne 1990). Data were collected from 31 August to 18 October; each colony was observed on alternating days over 22 days. All marked forager arrivals were recorded by noting worker identity and material carried during continuous observation periods of 1.5 h in the morning and 1 h in the afternoon. Foraging trip durations were measured at two colonies from 16 July to 8 August 1989. An observer seated at the nest recorded foraging behavior during 1 to 1.5 h continuous observation periods, one in the morning and another in the afternoon, daily or every two days. The observer noted worker identity, material carried, and the time to the nearest minute of all arrivals and departures of marked workers; from these data we calculated the time spent away from the nest by foragers, henceforth referred to as field time. In some cases workers flew from the nest but remained within a distance of 5 m, and returned to the nest within 1 min. These wasps appeared to be making orientation flights and were not counted as foragers. Results In order to increase statistical power, data on foraging trip success were pooled over the five observation colonies. The proportion offoragers making only successful trips increased with increasing foraging age (Fig. 1; Likelihood ratio Chi-Square test, G 2 = 13.43, df = 5, P < 0.05). The proportion of foraging trips that were successful, 2 pooled over individuals, also increased with increasing foraging age (G = 22.10, df = 5, P < 0.001). The proportion offoragers making only successful trips did not vary with calendar date (G2 = 22.52, df = 24, P < 0.50). Foraging trip duration data were analyzed using multiple regression to examine the effects of adult age and foraging age on time spent in the field. Only foragers that made trips on more than one day (n = 55 individuals, 761 foraging trips) were included in the analysis. The regression model included terms to account for variation in field time due to colony, calendar date, and material carried. The four materials collected by P. occidentalis foragers - nectar, water, insect prey, and wood pulp require different amounts of field time to collect a load (O'Donnell & Jeanne 1990). After regressing field time against the above factors, the residuals were regressed against foraging age and adult age. Time spent away from the nest on foraging trips Forager success in Po[ybia occidentalis 453 (8) (3) 1.0 --. (26) :; II) -II) 3 () (10) :::J II) 0.9 ~ :::J (54) -..II) CI> Q ..ca 0 u. 0.8 1:" D. (122) 3 5 7 9 11 Foraging age (days) Figure 1. The proportion ofmarked, known age foragers making only successful trips at different foraging ages. Numbers in parentheses are sample sizes increased with experience: field time bore a highly significant positive relationship with foraging age (F = 14.0, df = 1, P < 0.001), but not with true age (F = 0.Q1, df = 1, P > 0.75). Field time for each of the four materials was regressed individually against foraging age after correcting for colony ID and calendar date: only field time for nectar increased significantly (F 11.12, df = 1, P < 0.001); increase in field time for prey approached significance (F = 3.5, df = 1, P < 0.07); and field time for water and wood pulp did not change (F < 1.2, df = 1, P > 0.25 in both cases). Foraging rate (trips per observation hour) did not change significantly with foraging age for any of the four materials (Kruskal-Wallis tests, P > 0.10 in all cases). Discussion Our data support the hypothesis that forager performance as indicated by foraging suocess improved with experience in P. occidentalis. Foraging success rate increased with foraging age independently of calendar date, therefore directional changes in resource availability or colony need over time do not explain the patterns observed. Experience with foraging also led to an increase in time spent away from the nest, as was noted in desert ants by Schmid-Hempel and Schmid-Hempel (1984). Changes in worker age did not explain the patterns observed. The change in field times was primarily due to effects on food material foraging, which accounts for the majority of P. occidentalis colony foraging effort (O'Donnell and Jeanne 1992). Foragers that spent more time in the field may have collected larger loads than those making shorter forays, though we did not measure load sizes or other measures of load quality (such as nectar concentration) to test this possibility. 454 O'Donnell and Jeanne Alternatively, longer field times may reflect forager persistence, that is, experien ced foragers may be more likely to remain in the field until they locate and obtain a load. Foraging rate did not change with foraging age, therefore the increase in the proportion of fully successful foragers with foraging age demonstrates that forager reliability does increase with experience. However, longer field times may involve a cost to foragers: in an earlier study, we found that the probability of mortality increased with foraging age in P. occidentalis (O'Donnell & Jeanne 1992). If for agers undergo senescence after performance of a certain amount of foraging (as do honeybees; Neukirch 1982), the cost to the colony of increased risk of mortality due to persistence in the field may be low. A full understanding of the payoff to the colony of increased success in task performance will require simultaneously accounting for the benefits in material input and the risks of mortality associated with changes in worker behavior. Acknowledgements Our thanks to Dr. Larry Phelps, Cindy Butler, and Michael Evans for assistance in the field, and to the Costa Rican office of the Organization for Tropical Studies for logistical support. Werner and Lilly Hagnauers' permission to work on their property is gratefully acknowledged. The comments of an anonymous reviewer were valuable in improving the manuscript. Financial backing was provided by a grant from the University of Wisconsin-Madison Graduate School and NSF Grant BNS 8517519 to R.L.J., and a Tropical Fellowship from the Organization for Tropical StudiesfPew Charitable Trust to S. O'D. References Heinrich, B. 1976, The foraging specializations ofindividual bumblebees. Ecol. M onogr. 46: 05 -128. Jeanne, R. L., H. A. Downing and D. C. Post, 1988. Age polyethism and individual variation in Polybia occidentalis, an advanced eusocial wasp. In: R. L. Jeanne (ed) Inter individual behavioral variability in social insects. Westview press, Boulder, CO. pp. 323-357. Neukirch, A., 1982. Dependence of the life span of the honeybee (Apis mellifica) upon fight performance and energy consumption.